Optical fire detection systems including multiple flame sensors are known in the art. Exemplary systems are described in U.S. Pat. Nos. 6,518,574, 5,373,159, 5,311,167, 5,995,008 and 5,497,003. The flame sensors in such systems are typically equipped with a radiation detector and a unique optical filter that ranges from the ultraviolet to the infrared to allow for the measurement of the spectral content of objects within the flame sensor's field of view (FOV). By judiciously choosing the type of radiation detector, e.g., a Geiger-Mueller, a silicon, a pyroelectric, etc., in combination with the appropriately-specified optical filter for each radiation detector and electronically combining the output signals from the flame sensors, a flame can be discriminated from other innocuous sources. In this manner, based on the emissive characteristics of a flame and the anticipated false fire alarm sources, e.g., a radiant heater, cigarette, cigar, etc., within a monitored region a fire detection system can be developed by selecting the appropriate combination of radiation detectors and optical filters so that the anticipated false alarm sources does not cause a false alarm. In fire detection systems of this type, a fire alarm condition is identified and reported by the system when the sensed source of radiation appears to be spectrally similar to a flame as defined by the system designer and determined by the designer's choice of radiation detectors, optical filters and electronic combination of the resulting signals from the radiation detectors.
A shortcoming of optical fire detection systems of this type is manifested when a spatially small source of radiation is brought in close proximity to the flame sensors. That is because there is an inherent spatial disparity between the multiple flame sensors. This spatial disparity often results from the use of the discrete radiation detectors and can be directly measurable as a physical distance. Alternatively, this spatial disparity can result from the use of refractive, diffractive or reflective optical elements.
In particular, the radiation detector of each flame sensor has its own field of view that may not significantly overlap with that of an adjacent radiation detector until an object is several inches away from the radiation detector. If the spatially small radiation source is brought closer than the common field range of the radiation detectors, i.e., the range at which FOV of the radiation detectors overlap, a significant chance exists that one detector will observe more of the radiation source than any other radiation detector. As a result, the radiation detector that observed more of the radiation will have the chance to collect more radiation from the radiation source depending on the spectral characteristics of the radiation source and the optical filter associated with that particular radiation detector. Consequently, the electronic output from the flame sensor including that particular radiation detector could be skewed relative to the other flame sensors. Once received and analyzed, the information transmitted in the electronic output of that flame sensor could cause the fire detection system to trigger a false alarm.